Q: A neutron is moving with velocity u. It collides head on and elastically with an atom of mass number A. If the initial kinetic energy of the neutron be E , how much kinetic energy will be retained by the neutron after collision ?

(a) $ (\frac{A}{A+1})^2 E $

(b) $ \frac{A}{(A+1)^2} E $

(c) $ (\frac{A-1}{A+1})^2 E $

(d) $ \frac{A-1}{(A+1)^2} E $

Ans: (c)